The present invention relates to a seal for a sterilizable, tube, bag or pouch which is suitable for the construction of a bag or pouch into which an object may be introduced followed by the bag being sealed and sterilised using gamma radiation, ethylene oxide or steam at at least 125.degree. C. as well as being suitable for other sterilisation methods.
Several ways have been attempted for sealing bags so that they remain intact during sterilisation in which vacuum may be applied. Under vacuum and temperature the bag or pouch tends to inflate placing high stresses on any seals which are mechanically weakened by the high temperature. In accordance with one method a bag is formed from a first surgical grade sheet which impervious or substantially impervious to bacteria and similar infectious bodies but being pervious to sterilising gases such as ethylene oxide and including steam and a second impervious sheet made for instance from high density polythene. The two sheets can be joined together by a surface welding technique, that is in effect the two materials are locally callendered together under temperature and pressure so as to form a surface weld between the two dissimilar materials. The temperatures and pressures used are usually not sufficient to produce bulk melting of the sheet materials but sufficient surface welding an compaction of the materials occurs so that the seal produced is impermeable or at least no more permeable than the gas pervious layer. This type of weld preserves the basic structure of both materials even in the weld but suffers from low and variable tear strengths. The tear strength of such a surface weld may be under 1 N/mm and usually it is not possible to guarantee a tear strength of 0.6 N/mm at all places along the seal. One possible reason for this is that the gas pervious material is not as uniform as a solid gas impervious sheet and the seal strength variations are caused by this variability.
Another sealing method may be described as melt welding. In this procedure a weld is produced at a temperature such the two materials melt into each other, whereby the gas pervious material may absorb the molten mass of the impervious material or both materials may melt together. The temperatures and pressures may be such as to coalesce the two materials. This procedure creates a strong bond but has certain disadvantages. Often the impervious sheet material is highly oriented. This frozen-in strain may be released by melt welding such that the impervious material contracts. Due to the difference in the materials used for the impervious and pervious layers, the degree of contraction is different resulting in the seal buckling on one side. This buckling can be so severe that the seal folds over to form a total of four thickness of material adjacent the seal position. Because the internal strains are released by the welding process, such a seal may be described as a thermally relaxed melt welded seal. When a pouch made with such seals is finally sealed by a cross seal to form a closed bag, this double layer at the existing seals can cause difficulties in particular it can promote the formation of very small diameter pores or openings close to the folded over seal. This problem can be avoided by the use of specialised welding techniques and apparatus but this is perceived as a disadvantage by some.
It is an object of the present invention to provide a seal for a sterilizable pouch or bag which can withstand the pressures generated by steam sterilisation at 125.degree. C. and above.
It is a further object of the invention to provide seal for a sterilizable pouch which can be securely manufactured relatively easily.